Fiber shattering process

ABSTRACT

A high-velocity jet stream of textile-length fibers is caused to impinge onto a rigid airfoil at a velocity above the critical velocity at which the viscoelastic flow property of the fibers can exert itself, with a consequent shattering of a substantial portion of the fibers into fibers of shorter length. The shorter length fibers may be decelerated and diffused into a plenum chamber, from which they may be removed in the form of a fibrous web.

United States Patent [191 Marshall 1 May 15, 1973 [54] FIBER SHATTERING PROCESS [75] Inventor: Preston F. Marshall, Walpole, Mass.

[73] Assignee: The Kendall Mass.

22 Filed: Aug 9, 1971 21 Appl.No.: 169,941

Company, Boston,

[52] US. Cl ..19/.3,19/156.3

[51] Int. Cl. ..D0lg 1/02 [58} Field of Search ..19/.3,155, 156.1, 19/1563; 65/5, 7, 9,11

{56] References Cited UNITED STATES PATENTS 2,365,970 l2/1 944 Pearce ..19/155 X 2,968,069 1/1961 Powell l9/156.l

FIBER RESTRAINT Primary Examiner-Dorsey Newton AttorneyJohn F. Ryan [57] ABSTRACT A high-velocity jet stream of textile-length fibers is caused to impinge onto a rigid airfoil at a velocity above the critical velocity at which the viscoelastic flow property of the fibers can exert itself, with a consequent shattering of a substantial portion of the fibers into fibers of shorter length. The shorter length fibers may be decelerated and diffused into a plenum chamber, from which they may be removed in the form of a fibrous web.

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FIBER SHATTERING PROCESS This invention relates to a process for converting viscoelastic textile filamentary material into fibers of shorter length, and to the collection of the fibers thus formed into a web or fleece. More particularly it relates to a process wherein a high velocity fluid stream of filamentary material is directed against an airfoil under conditions such that the normal viscoelastic behavior of the filaments cannot exert itself, with a consequent shattering of the filaments.

In the formation of fibrous webs to be fashioned into nonwoven fabrics, batting, coil, and the like, it is customary to feed a supply of textile-length fibers to a card, garnett, air-lay machine, or the like, which separates discrete and small groups or clumps of fibers from the feed mass and assembles these discrete clumps in the form of a fibrous web.

There are at least two disadvantages in the production of such fibrous webs by conventional methods. First, man-made fibers are produced in the form of a bundle or tow of continuous filaments, which must be cut to staple length by a tow cutter, for formation into card webs. Second, and more importantly, the conventional web-forming devices mentioned above do not separate fibers completely from each other, but instead they pluck off small groups or clumps of fibers from the main fibrous mass, so that card webs and gamett webs have a blotchy appearance. Furthermore, a fibrous web composed of fibrous clumps or aggregates does not develop the tensile strength which it would possess if the fibrous elements were substantially completely separated from one another and then reassembled in web form.

It has now been found that if a high-velocity, lowpopulation density air stream of viscoelastic fibers is directed against a substantially rigidly fixed airfoil, under certain conditions set forth below the filaments or long fibers are shattered into shorter lengths, and are dispersed in the air stream as individual fibers substantially free from entanglement with each other. For the formation of a fibrous web, the air stream carrying the short fibers is diffused into a plenum chamber, where expansion decelerates the stream velocity and from which the fibers may be drawn off as a continuous web or fleece.

By visco-elastic fibers is meant herein those organic fibers which show a non-Hookean behavior in their stress-strain curves, the curves being non-linear in regions where primary or secondary creep or plastic flow take place, indicating internal adjustment to forces tending to rupture the fiber. Such fibers, visco-elastic in nature, commonly have an elongation at break of greater than percent, and an average stiffness index of under 100, where the stiffness is expressed as breaking tenacity per unit of elongation. Unlike glass fibers and other inorganic fibers, visco-elastic organic fibers can absorb considerable energy without rupture.

By airfoil is meant herein any obstruction, such as a smooth bar, capable of producing reaction in a stream of air moving past it.

It has long been appreciated that the stress-strain be havior of a textile yarn depends on the rate at which the stress is applied. This has led to studies in which the stress, instead of being applied gradually, is applied to the yarn by ballistic impact (Kaswell, Wellington Sears Handbook of Industrial Textiles, Wellington Sears Company, Inc., New York, 1963, page 483 et seq.). Normally, stress applied to a viscoelastic fiber causes a plastic flow of the fiber substance, the fiber adjusting by deformation, due to its visco-elastic nature. As the velocity of stress application increases, a textile fiber will usually show an increased breaking strength and a decreased elongation, since visco-elastic readjustment is time-dependent. It has been postulated by von Karman and others that there exists a critical velocity of applied stress, as by impact, above which the timedependent visco-elastic property of textile fibers has no time to function. There is no energy absorption by plastic flow, and the fiber breaks if the instantaneous elastic deflection value is exceeded. Considerations such as these have been used in the study of the impact of ballistic missiles on textile fabrics, and in the design of body armor.

In the process of this invention, the missile-fiber concept is reversed. An airborne stream of viscoelastic fibers is beamed at a rigid airfoil at a velocity above the critical velocity at which viscoelastic flow can exert itself, said critical velocity ranging from 300 to 1,500 feet per second depending on the particular stressstrain behavior of the fibers concerned.

It is a primary object of this invention to provide a process for cutting viscoelastic textile fibers into shorter but still preponderantly textile lengths by ballistic impact of an airborne stream of long fibers against a rigid airfoil.

It is another object of the invention to provide a process for the conversion of an air-borne stream of viscoelastic textile fibers into a fleece or web of shorterlength fibers.

The invention will be better understood with reference to the following description and drawings, in which:

FIG. 1 is a cross-sectional representation of a high velocity, highly attenuated fluid stream of fibers being delivered from a nozzle, principally broken away, and impinging on an airfoil.

FIG. 2 is a cross-sectional side view of the fibrous stream impinging on an airfoil mounted in a plenum chamber.

FIG. 3 is a front elevation of the plenum chamber of FIG. 2.

FIG. 4 is a cross-sectional side view of a device for forming a high velocity fibrous stream by means of an aspirator, in which a strand of textile-length fibers is attenuated by a one-step process.

FIG. 5 is a similar cross-sectional view partly broken away of a device wherein a high velocity fibrous stream is formed by a two-step process.

Various methods of forming a high velocity, attenuated fluid stream of fibers by means of an aspirator will occur to those skilled in the art. Two exemplary devices are disclosed in my copending U.S. Pat. applications Ser. No. 159,229, filed July 2, 1971, and Ser. No. 164,255, filed July 20, 1972. In the former application as shown in FIG. 4, a twist-free strand 60 of textilelength fibers is fed to an-aspirator tube 64, maintained substantially constant in cross-sectional population through a substantial portion of the aspirator tube, and then drafted into an attenuated, high velocity fibrous stream 68 by the action of a fluid stream such as high velocity air, which expels the drafted and attenuated strand from the aspirator.

In Ser. No. 164,255, a two-stage process is described wherein the leading end of a fibrous strand 80 is subjected to an initial drafting and attenuation by the effect of a vacuum established at the entrance end 84 of an aspirator tube, accelerating and attenuating the resultant stream of air-borne fibers through the body of the aspirator, and further accelerating and attenuating the fibrous stream 86 by the action of a second highvelocity air stream at or near the exit end of the aspirator. The parameters of the two processes, and the design elements of aspirators suitable for creating attenuated high-velocity fibrous streams, are set forth in detail in said Ser. Nos. 159,229, and 164,255, which are hereby incorporated by reference.

It will be obvious that these methods are exemplary and not restrictive, other procedures for obtaining high velocity air streams of low fiber population density being known.

Referring to FIG. 1, a fluid stream of fibers 10 is shown as issuing from the orifice 12 of a jet nozzle 14. The design of a suitable nozzle, and the conditions needed to form the fibrous stream 10, are set forth in my applications Ser. No. 159,229 and Ser. No. 164,255 referred to above. As explained therein, the stream of fibers may be formed either by the pressure of a jet stream of near sonic velocity, or by a vacuum developed at the nozzle entry.

The stream of long viscoelastic fibers l issuing from the jet nozzle is characterized by a high velocity, preferably in the range of 1,000 feet per second or more, and by a low population density per cross-section. That is, the attenuation of the fibrous strand in the nozzle is of a high order of magnitude. In a typical example, a viscose rayon sliver of 191, 317 denier, 34,785 filaments, was fed to the nozzle at a rate of 11.7 feet per minute. The calculated velocity of the air stream in the divergent portion of the nozzle was 1,000 feet per second, and the number of fibers in the cross-section of the stream dropped from 34,785 to an average of less than 7, an attenuation factor of over 5,000 times.

A high degree of attenuation, or draft factor, is needed in the process of this invention in order that each fiber may be substantially freed from frictional engagement with any other fiber. By employing a draft factor of 1,000 or higher, and velocities in the range of 1,000 feet per second or higher, the viscoelastic fibers are individualized, with a consequent enhanced efficiency in the shattering step and an enhanced uniformity of the web or fleece made from the resultant shorter fibers.

Referring still to FIG. 1, the high-velocity stream 10 of long fibers is beamed at an airfoil 16, located downstream at a distance which is governed by the length of staple desired in the effluent stream 18 of short fibers. In general, when a cylindrical airfoil is used, its diameter may be from two to six times the diameter of the jet stream 10, and it is preferably located from four to 10 times the diameter of the jet stream downstream from the nozzle mouth 12. If the jet stream 10 of fibers is composed of individualized rayon filaments of about 6 inches in length, locating the airfoil 16 at about 4 inches from the outlet 12 of the nozzle will result in an effluent stream 18 wherein the average fiber is approximately 0.5 inches in length when the velocity of the jet stream 10 is 800-1,000 feet per second.

The fiber length in the effluent stream 18 will of course vary with the number of long fibers in a crosssection of the jet stream 10, and hence with the pounds per hour production rate, fiber length increasing as more fibers per hour are processed. By increasing the velocity of the jet stream 10, the fiber length of the effluent is decreased. And by moving the airfoil farther from the nozzle, the fiber length of the effluent is increased, all other factors remaining fixed. For most of the purposes of this invention it is preferred that the preponderance of the shattered fibers be of textile length, defined as 0.25 inches and longer, in distinction to the shorter fibers characteristic of the paper art.

The airfoil 16 may vary widely in design and contour, from streamlined teardrop shape to the convenient simple cylindrical section of pipe as shown. The airfoil performs a dual function. Primarily, it serves as an impingement target against which the long fibers are shattered, due to their inability to exhibit plastic readjustment. Under preferred conditions of operation, each long fiber is shattered into a multiplicity of shorter fibers.

A second function of the airfoil is to convert the jet stream of long fibers, circular in cross-section into a broad turbulent fan of air-borne short fibers at substantially lower velocity. In this condition, as shown in FIG. 2, the short fiber stream may be further decelerated and diffused by expansion into the plenum chamber 20.

Referring to FIG. 2, there is shown a plenum chamber with the side panel removed. The turbulent flow of short fibers 18 is further diffused and decelerated by expansion into the upper chamber 30. Although various types of plenum chamber may be used, for convenience and economy of space the device as shown is divided into an upper, middle, and lower chamber 30,32, and 34 respectively, separated by the plates 42, 44, and 46 which extend the full width of the chamber. Plates 42 and 44 do not extend the full length of the chamber, however, so that the path of the decelerating fibers is a serpentine one, as shown by the dotted line. The rear and front partitions, 36 and 38, act as baffles, deflecting the air stream, so that a constant and substantially uniform flow of fibers, at a manageable velocity, emerges from the plenum chamber exit 48 to impinge on the upper surface of the screen conveyor belt50. If desired, a conventional vacuum box 52 may be mounted on the under side of the porous conveyor to bleed off the last traces of air and assure proper deposition of the fibrous web 54 on the conveyor.

In some cases, where the web is to be subjected to subsequent treatment such as bonding or impregnation, it may be found that very short-fibered webs have insufficient strengthto survive the rigors of wet processing. In such instances it is convenient to interpose between the conveyor screen and the fibrous stream a layer of permeable supportive material such as gauze, cellulose tissue, porous nonwoven fabric, or the like. Such expedients are well-known in the art, and are not shown.

The term manageable velocity" as employed above means a stream velocity at which the fibers can be deposited continuously onto a moving porous belt with substantial absence of fiber clumping or deflection of the stream. The purpose of the plenum chamber or diffuser, therefore, is to spread the high speed jet stream over a large cross section, so that the kinetic energy of the stream is transferred to pressure, by diffusion. This pressure forces the air through the porous conveyor, which filters out the fibers in web or fleece form. A convenient range of exit velocity that is, the air velocity at which the decelerated fibrous stream impinges on the porous conveyor belt has been found to be 3 to 30 feet per second. As mentioned above, the critical velocity for the ballistic shattering of long fibers by impingement on an airfoil has been estimated at 300 to 1,500 feet per second. A preferred range for viscose rayon fibers involves a jet stream velocity of 800 to 1,200 feet per second. Throttling this velocity down to a managable exit web velocity is a function of the parameters of the plenum chamber, which can be calculated from a consideration of the volume of air to be handled.

The invention will be illustrated by the following examples.

EXAMPLE 1 A three-section plenum chamber was used, as in FIGS. 2 and 3. The upper chamber 30 was 20 inches square in cross-section, with a circular opening 40 of 16 inch diameter. The middle chamber 32 was 30 inches wide and 6 inches deep, while the lower chamber was 40 inches wide and 4.5 inches deep. The length of the plenum chamber was 40 inches. The airfoil 16 was a cylindrical section of polished steel pipe 1.75 inches in diameter, mounted inside the opening 40 into the plenum chamber, at a distance of 6 inches from the outlet 12 of the jet nozzle 14.

A 5.5 denier per filament viscose rayon top, out 6 inches long and pin-drafted for parallelization, was fed to an air jet of Type A according to the procedure set forth in my copending application Ser. No. 159,229. The input feed to the nozzle was 191,317 denier, 34,785 filaments, the operating air pressure was 125 P816, and the strand was observed to substantially fill the aspirator.

The resultant jet stream of individualized 6 inch fibers contained on the average 4.58 fibers per cross section, the draft factor being 7,595 as calculated from the input top speed of 7.9 feet per minute and the jet output speed of 60,000 feet per minute, or 1,000 feet per second.

The expanded fibrous stream resulting from diffusion of stream 18 in plenum chamber was collected on a layer of open-meshed gauze carried on the 40 inch conveyor screen 50. The web weighed 6.00 grams per square yard, or 75.6 square yards per pound, and was produced at a rate of 23 feet per minute.

The length of the average fiber in the web was approximately 0.5 inches, indicating that on the average, each 6 inch fiber in the jet stream 10 had been shattered into a dozen shorter fibers. As set forth above, the length to which the long fibers are shattered may be adjusted as desired by changing the velocity of or the number of fibers in the jet stream, or by changing the placement of the airfoil.

EXAMPLE 2 Using the same plenum chamber as in Example 1, and the same fibrous strand, the strand was passed through an aspirator of Type B as set forth in Ser. No. 164,255 operating at an air pressure of 125 PSIG at a rate of 21.6 feet per minute or 18.6 pounds per hour. The operating air pressure was sufficient 'to develop a static vacuum of about 23 inches of mercury at the entrance end of the aspirator tube, and the strand, restrained by feed rolls at a distance of 7 inches from the entrance of the aspirator, was seen to be attenuated and drafted into a thinner stream of fibers as it entered the aspirator.

The high velocity stream of fibers 10 emerging from the aspirator exit section 12 was caused to impinge on the same airfoil, mounted similarly as in Example 1, in the plenum chamber, but the exit end of the aspirator was held at a distance of 7 inches from the airfoil.

The resultant fibrous stream 18 from the impingement of the fibrous stream 10 on the airfoil 16 was diffused into the plenum chamber and collected on a layer of open-meshed gauze carried on the conveyor screen 50. The web weighed 65.3 grams per square yard and was produced at a rate of 5.78 feet per minute.

The majority of the fibers varied from 2 inches to nearly 6 inches long, indicating a less severe degree of fiber cutting due presumably to the greater distance between the exit section of the nozzle and the airfoil.

Fibrous webs produced by the process of this invention are of remarkably uniform density and opacity, and may be made substantially free from the streaks and light spots which are characteristic of carded or garnetted webs. This is due to the unique nature of the process, in that each fiber is carried in the air stream as a single fiber, and the fibers are not clumped and entangled into irregularly shaped clusters as they are in a mechanical carding action.

In part also this uniformity is the result of the fact that webs produced by the process of this invention comprise fibers of varying length. For example, the web of Example 1, although composed principally of halfinch fibers, contained longer fibers as well as an appreciable proportion of shorter fibers which infiltrated into the basic fiber network. The shattering of long fibers into shorter lengths by ballistic impact against an airfoil is a random event, unlike conventional tow cutting, due to the random nature of impingement of the fibers against the airfoil, and perhaps to microspopic variations in structure along the length of the long fibers.

Having thus described my invention I claim: 1. The process of forming a fluid-home fibrous stream of fibers substantially of textile length which comprises forming a multiplicity of longer organic viscoelastic textile fibers into a high-velocity fibrous jet stream, the viscoelastic fibers having an elongation at break of greater than 10 percent and a stiffness index of not over 100, stiffness index being expressed as breaking tenacity per unit of elongation,

an impinging said jet stream onto a substantially rigid airfoil at a velocity of at least 300 feet per second and sufficient to cause a shattering of a major portion of said fibers into a stream of fibers of shorter length.

.2. The process according to claim 1 in which the high velocity fibrous jet stream is a stream of low population density, attenuated from a fibrous strand.

3. The process according to claim 2 wherein the highvelocity fibrous jet stream is formed by feeding a fibrous strand to an aspirator tube, maintaining the strand substantially constant in cross-sectional population through a substantial portion of said aspirator tube, and then drafting said strand into an attenuated, high velocity jet stream by the action of a fluid stream such as an air stream.

4. The process according to claim 2 wherein the highvelocity fibrous jet stream is formed by subjecting a fi- '7 I 8 brous strand to an initial drafting and attenuating opercoelastic fibers are viscose rayon fibers. ation by the effect of a vacuum established at the en- 7. The method of forming a fibrous web of predomitrance end of an aspirator tube, accelerating and attennantly textile-length fibers which comprises uating the resultant stream of air-borne fibers through forming a fluid-borne fibrous stream of fibers subthe body of the aspirator tube, and then further accelstantially of textile length according to the process crating and attenuating the fibrous stream by the action of claim 1, of a second high-velocity air stream. decelerating and diffusing said fibrous stream by ex- 5. The process according to claim 1 in which the pansion of said stream into a plenum chamber, high-velocity jet stream of fibers has a velocity of beand collecting the fibers from said plenum chamber tween 300 and 1,500 feet per second. 10 in the form of a fibrous web.

6. The process according to claim 1 in which the vis- 

1. The process of forming a fluid-borne fibrous stream of fibers substantially of textile length which comprises forming a multiplicity of longer organic viscoelastic textile fibers into a high-velocity fibrous jet stream, the viscoelastic fibers having an elongation at break of greater than 10 percent and a stiffness index of not over 100, stiffness index being expressed as breaking tenacity per unit of elongation, an impinging said jet stream onto a substantially rigid airfoil at a velocity of at least 300 feet per second and sufficient to cause a shattering of a major portion of said fibers into a stream of fibers of shorter length.
 2. The process according to claim 1 in which the high velocity fibrous jet stream is a stream of low population density, attenuated from a fibrous strand.
 3. The process according to claim 2 wherein the high-velocity fibrous jet stream is formed by feeding a fibrous strand to an aspirator tube, maintaining the strand substantially constant in cross-sectional population through a substantial portion of said aspirator tube, and then drafting said strand into an attenuated, high velocity jet stream by the action of a fluid stream such as an air stream.
 4. The process according to claim 2 wherein the high-velocity fibrous jet stream is formed by subjecting a fibrous strand to an initial drafting and attenuating operation by the effect of a vacuum established at the entrance end of an aspirator tube, accelerating and attenuating the resultant stream of air-borne fibers through the body of the aspirator tube, and then further accelerating and attenuating the fibrous stream by the action of a second high-velocity air stream.
 5. The process according to claim 1 in which the high-velocity jet stream of fibers has a velocity of between 300 and 1,500 feet per second.
 6. The process according to claim 1 in which the viscoelastic fibers are viscose rayon fibers.
 7. The method of forming a fibrous web of predominantly textile-length fibers which comprises forming a fluid-borne fibrous stream of fibers substantially of textile length according to the process of claim 1, decelerating and diffusing said fibrous stream by expansion of said stream into a plenum chamber, and collecting the fibers from said plenum chamber in the form of a fibrous web. 